The present invention relates to the field of cellular telephone systems, and more particularly to a method for detecting the presence of a dotting sequence for a Manchester encoded signal in a deep fading environment.
Cellular telephone systems are widely used today throughout the world. Such systems are implemented either as analog or digital systems. In certain analog and digital systems, data is encoded using an encoding scheme known as Manchester encoding in which each nonreturn-to-zero binary one is transformed to a zero-to-one transition, and each nonreturn-to-zero binary zero is transformed to a one-to-zero transition. The ability to correctly detect Manchester encoded data is crucial to the proper operation of conventional cellular telephone systems.
Manchester encoded data signals present a single transition edge when the cellular signal is comprised of alternating ones and zeros as shown in FIG. 1A, or two transition edges when the signal is comprised of a sequence of all ones or all zeros as shown in FIG. 1B. In the latter case, the edges coinciding with the vertical dashed lines must be ignored in order for a cellular telephone receiver to receive the cellular signal.
Conventional cellular telephone systems employ a fixed frequency tone comprised of an alternating sequence of ones and zeros, i.e., 10101010, known as a dotting sequence to initialize the phase of the clock of the cellular receiver. Upon receipt of the dotting sequence, the receiver locks to one transition edge of the cellular signal waveform and generates a mask pulse to mask the other edge of the signal. The masked edge is 180 degrees out of phase with the locked edge. If at the end of the dotting sequence the receiver is locked to the correct edge, it can receive the cellular signal without having to shift the phase of its clock. However, if at the end of the dotting sequence the receiver is locked to the incorrect edge of the signal such that the correct edge is masked and no transition edges have appeared for a predetermined number of consecutive clock cycles which are shorter in duration than the dotting sequence, then the receiver can not receive the cellular signal, and it shifts the phase of its clock by 180 degrees so that it can lock to the correct edge and receive the cellular signal.
Conventional cellular receivers suffer from a significant drawback. Specifically, in environments where deep fading of a cellular signal occurs such that the signal can not be received, conventional cellular receivers interpret the absence of edge transitions as indicating that a dotting sequence is present and that the receiver locked to the wrong edge. In response the receiver shifts the phase of its clock by 180 degrees. Consequently, if the receiver was locked to the correct edge of a signal before the deep fading, by shifting its phase 180 degrees, it will then be locked to an incorrect edge of the signal once the deep fading ceases. The receiver will therefore be rendered inoperable, being unable to receive any intelligible data until the next dotting sequence reinitializes its clock.
A method for a cellular telephone receiver to detect the presence of a dotting sequence for a Manchester encoded cellular signal in a deep fading environment, wherein both the presence of a single edge transition during the mask pulse and the absence of any transition edges outside of the mask pulse for a predetermined number of consecutive clock cycles, indicate the presence of a dotting sequence and that the receiver locked to a masked edge, thereby preventing the receiver from receiving the signal. In response, the receiver will shift the phase of its clock by 180 degrees so that it can lock to an unmasked edge of the cellular signal and thereby receive the signal.
The absence of any transition edges or the presence of more than one transition edge during the mask pulse indicates that the receiver is not receiving the cellular signal because of deep fading and not because it locked to a masked edge of the signal during the dotting sequence. In response, the receiver will not shift the phase of its clock, but will instead remain locked to an unmasked edge so that it can receive the cellular signal once the deep fading ceases.